A conventional process for manufacturing a link plate for a silent chain is depicted in FIGS. 1 to 7 of Japanese Laid-open Patent Publication No. 2002-28749. In the conventional process, after the outline of the meshing tooth surface is punched from a band of sheet steel, the inside tooth surfaces and the crotch portion of the link plate are simultaneously shaved, so that the surfaces are made uniformly smooth. As a result, the formation of microscopic cracks is prevented, and the fatigue strength of the link plate is enhanced.
Conventional processes for manufacturing a link plate for a silent chain, are illustrated in FIGS. 5 to 7 of this application.
As shown in FIG. 5, in the formation of a particular link plate 500, rough punching and shaving are performed by a die assembly D, on a band H of sheet steel, which is intermittently forwarded past the die assembly. First, connecting pin holes are punched by dies d1 to form inner hole surfaces 510. Then, the steel band is indexed forward and the connecting pin holes are shaved by shaving dies S1. In the next step, the meshing tooth surfaces 520, which are shaped to mesh with sprockets, are formed by punching die d2. The meshing tooth surfaces are then shaved by shaving die S2. Following the shaving of the meshing tooth surfaces the link plate back surface 530, which slides on the shoe surface of a chain guide, is formed by punching die d3. Then the back surface 530 is shaved by die S3. Finally the link plate 500 is separated from the steel band by die dn.
After the band H is pulled from a supply roll (not shown) it is intermittently forwarded into the punching and shaving die assembly D, using pilot holes R as a reference. After punching, the remainder of the band H is recovered by winding it onto a take-up roll (not shown). A very slight bending or strain generated as the band H is wound onto the supply roll causes snaking of the band H as it is forwarded past the die assembly. As shown on a larger scale in FIG. 6, which is an enlarged view of a region B in FIG. 5, the snaking movement of the band H results in a lateral displacement of the band by a distance X as it moves from the location at which the meshing tooth surfaces are formed by punching die d2, to the location at which the same meshing tooth surfaces are shaved by shaving die S2. The displacement X increases if the shaving die S2 in the die assembly D is closer to the take-up roll.
In the case of a relatively small shaving margin, e.g., a margin of about 0.1 mm, lateral displacement of the band H can cause a portion of the meshing tooth surface not to be shaved. As a result, the desired shear surface ratio and surface roughness may not be obtained, and the wear elongation of the chain is increased. Furthermore, in the portions of the link plate tooth surface where shaving is not performed, microscopic cracks are generated. Chain breakage occurs due to the microscopic cracks, and the required chain strength cannot be maintained for a long period of time. Additionally, whereas ring-shaped chips would ordinarily be produced and discharged in the case of perfect alignment between the shaving die and the roughly punched link plate, the punching displacement X causes the chips to be only partially cut off, and the partially cut off chips are not perfectly discharged. Parts of the chips remain as flaws on the link plates 500, reducing the quality of the link plates.
An additional problem is that, because of the bending of the band H as it moves past the die assembly D, the accuracy of the distance between the punched inner surfaces 510 of the connecting pin holes and the punched meshing tooth surface 520 cannot be maintained. The variations in the distance between the connecting pin holes and the meshing tooth surfaces result in variations in meshing of the chain with the sprocket, which leads to a reduction in chain strength.
A further problem encountered in the conventional method of forming silent chain link plates was that the accuracy of the distance between the inner surfaces 510 of the connecting pin holes and the back surface 530 of the plates could not be maintained. These distance variations result in variations in the positions of the plate back surfaces 530 relative to the chain travel line, causing accelerated wear of the shoe of a chain guide.
Another conventional method of manufacturing a link plate 600 for a silent chain is shown in FIG. 7. Rough punching of the connecting pin holes is first carried out. Then, their inner surfaces 610 are shaved. After punching and shaving of the connecting pin holes, the meshing tooth surfaces 620 and the link plate back surface 630 are punched simultaneously. Then the meshing tooth surface 620 and the plate back surface 630 are shaved simultaneously. The same problems as experienced in the process depicted in FIGS. 5 and 6 are experienced in the process of FIG. 7. Snaking of the band H of sheet steel causes lateral displacement of the band between the punching and shaving stations, resulting in incomplete shaving, the formation of microscopic cracks, and variations in the distances between the connecting pin holes and the meshing and back surfaces of the link plates.
The objects of the invention are to solve the above-described problems experienced in conventional processes of forming link plates for silent chains. More particularly, an object of the invention is to suppress displacement of the band of sheet steel between the shaving and punching stages even when bending of the band occurs as it is forwarded past the punching and shaving die assembly. Other objects of the invention include improvement in the uniformity of the link plates, improvement in finishing accuracy of the meshing tooth surfaces, the back surfaces of the link plates, and the connecting pin holes, achievement of an accurate parallel relationship between the outer peripheral surfaces of the link plate and the inner peripheral surfaces of the connecting pin holes, and achievement of an accurate perpendicular relationship between the flat faces of the link plates and the outer and inner peripheral surfaces formed by punching.